In general, when four-wheel drive is engaged, a control unit checks the sensors and actuator. If there is nothing wrong, a front speed sensor detects the speed of the driving axle that supplies power from the engine to a magnetic clutch. A rear speed sensor detects the speed of a driven axle to which power is conveyed from the driving axle through the magnetic clutch. A comparison is made as to whether the rotational speed of the driving axle sensed by the front speed sensor is greater than that of the driven axle sensed by the rear speed sensor.
If the speed of the driving axle is not greater than that of the driven axle, a comparison is made as to whether the rotating speed of the driven axle is larger than that of the driving axle. If the speed of the driving axle is greater than that of the driven axle and if the speed of the driven axle is greater than that of the driving axle, the control unit appropriately adjusts the magnetic clutch slip to less than a predetermined level of allowable slip and adequately supplies power from the driving axle to the driven axle, thereby actuating the four-wheel drive.
However, there are drawbacks in conventional control methods for four-wheel drive. When the magnetic clutch of the vehicle is overloaded, there may be a big difference in the rotational speed of the driving and driven axles, even if the magnetic clutch is controlled at its maximum duty, thereby causing continuous excessive slip and bringing about burning due to overheating of the magnetic clutch.